The present disclosure relates to an oscillator circuit that controls an oscillation frequency with the use of a digital signal, and to a frequency synthesizer that includes such an oscillator circuit.
A frequency synthesizer is often mounted on a semiconductor device in order to generate a signal having a desired frequency. The frequency synthesizer may be achieved by an analog phase locked loop (PLL), for example. The analog PLL includes a voltage controlled oscillator (VCO) circuit that is capable of causing an oscillation frequency to vary in correspondence with an analog control voltage. In such an analog PLL, a control voltage is so controlled as to cause the oscillation frequency to be constant.
In recent years, generations of a semiconductor manufacturing process have progressed, pressure resistance of a device is decreased, and a power source voltage of a semiconductor device is decreased accordingly. As a result, in the analog PLL, a voltage range of the control voltage of the voltage controlled oscillator circuit has been narrower, and it has therefore been more difficult to generate an oscillation signal having a wide frequency range. Accordingly, a digital PLL has been often used that includes a digitally-controlled oscillator (DCO) circuit that controls an oscillation frequency with the use of a digital signal. For example, Japanese Unexamined Patent Application Publication No. 2009-10599 discloses a DCO circuit that controls frequencies of a high-order capacitor bank and a low-order capacitor bank based on a frequency control word including a fractional bit and an integer bit.